When undefined parameters are used in a claim the question arises whether the skilled person is able to determine whether he is working within or outside the scope covered by said claim and whether this aspect is a matter of sufficiency of disclosure according to Article 83 EPC or just clarity according to Article 84 EPC.
Subsuming said question under Article 83 or Article 84 EPC is of outmost importance as in opposition proceedings the patent can be examined for its compliance with Article 83 EPC, however not with Art. 84 as long as no claim amendment has been made. Unfortunately the case law of the Boards of Appeal of the EPO is still inconsistent in this regard (see e.g. C. Keussen, “Zum Verhältnis von Art. 83 und 84 EPÜ im Einspruchsverfahren – alles klar?” GRUR 2014, 132-137) and depending on which side you are standing on in opposition proceedings you may support your position with decisions in favor of Article 83 or Article 84 EPC.
Recent EPO’s Board of Appeal decision T 89/13 of May 3, 2016 is a further decision in favor of Article 84 EPC and points out that once the question boils down to the argument that the boundaries of the claims are not clearly defined, then this is a matter of clarity (Art. 84 EPC) of the claimed subject-matter, not sufficiency of disclosure (Art. 83 EPC).